(1) Field of the Invention
The present invention relates to an active control system, for actively reducing noises in a vehicular compartment and/or vehicular body vibrations, applicable to an automotive vehicle in order to enhance the riding comfortability.
(2) Description of the Background Art
A previously proposed kind of active control system is exemplified by a British Patent Application Publication No. 21 49 614.
The previously proposed active control system disclosed in the above-identified British Patent Application Publication is applied to an actively noise controlling system for an enclosed space such as a vehicular compartment or cabin of an airplane.
That is to say, a number of loud speakers and microphones are installed within the enclosed space. The loud speakers serve to generate control sounds to be interfered with noises and tile microphones serve to measure a residual error signal (residual noise). A signal processor is connected to the loud speakers and microphones. The signal processor receives a basic frequency of a noise source measured by basic frequency measuring means and input signals from the microphones and outputs control sound signals to the loud speakers so as to minimize a sound pressure level within the enclosed space. Although three loud speakers and four microphones are installed within the enclosed space, it is now assumed that the numbers of the loud speakers and the microphones are single, respectively, in order to simplify the explanation of the active noise controlling system.
Suppose now that a transfer function from the noise source to the microphone is H, a transfer function from the loud speaker to the microphone is C, and a sound source information signal generated by the noise source is X.sub.p, the residual error signal E is expressed as: EQU E=X.sub.p .multidot.H+X.sub.p .multidot.G.multidot.C
It is noted that, in the above equation, G denotes a transfer function required to cancel the noise sound.
When the noise is perfectly canceled at a point of location at which the noise is to be canceled, E=0.
At this time, G=-H/C.
Then, the transfer function G is determined which minimizes the microphone detected signal E and filter coefficients of a filter disposed in the signal processor are adaptively updated on the basis of the transfer function G. Means for deriving the filter coefficients so as to minimize the microphone detected signal E includes an LMS (Least Mean Square) algorithm which is a kind of a steepest descent method.
When the number of microphones are installed within the enclosed space, the noises are controlled so as to minimize a total sum of the signals detected by means of the respective microphones.
According to the control procedure described above, a canceling sound is output having a phase opposite to the enclosed noise in the vehicular compartment so that the canceling sound cancels the enclosed noise.
The above-described previously proposed active noise controlling system is effective in a case where a single noise source generating stationary deterministic signals such as a sinusoidal signal component, for example, a noise generated from an engine mounted in the automotive vehicle.
However, the noises in the vehicular compartment during the vehicle run include various types of noises such as a noise involved in engine revolutions, so-called road noise transmitted from vehicular suspensions, so-called wind noise generated at a vehicular exterior during the vehicle run, exhaust noise and, in this addition, these noises are random characteristics. Hence, in the previously proposed noise controlling system, the noise control is carried out with the sinusoidal (sin) signal component of the noise involved in the engine revolutions as a reference signal, noise components except an object to be controlled are added whenever the calculation of the noise canceling sound is carried out so that a divergence of control would be brought out. In this case, an increase of noise level would often be brought out.
In addition, although the active control system described above is applicable to a vibration control system for the vehicular suspensions other than the actively noise controlling system, vibration inputs to vehicular suspension units have random characteristics. It is difficult for the previously proposed active control system to appropriately control vibrations of such vehicular suspension units as described above.